Wednesday, May 18, 2016

Aristotle, Einstein, and the nature of force

But Newton also removed Aristotles division of the motions into “natural motions” and “enforced motions”. For Newton, also Aristotles “natural motions” became “enforced” by the gravitational force. In this way, he unified our understanding of dynamics in the most general way. ...

In 1915, Albert Einstein had found the basic equation in his theory of general relativity. He published a complete version of his thoughts in 1916. According to this theory, the gravitational interaction was not caused by a force but by a curvature of spacetime. In this basic publication Einstein writes: “Carrying out the general theory of relativity must lead to a theory of gravitation: for one can generate a gravitational field by merely changing the coordinate system.” ...

To summarize, let us state that motion caused by gravitation is not caused by a force; in that sense it differs from all other motions. Einstein made this clear in his quoted paper from 1916. He writes:21 “According to the theory of General Relativity gravitation has an exceptional role with respect to all other forces, especially electromagnetism.” This is what in a sense we might call “return of Aristotelian physics” since it clearly distinguishes between “natural motion” and “enforced motion”, constituting the basic problem of modern physics.

Acceleration is either caused by the geometry of spacetime (gravitation) or by an external force in Euclidian spacetime (all other forces). Mathematically, these two different views are represented either by the Theory of General Relativity (gravitation) or by Quantum Field Theory (all other forces).

This is partially correct. Aristotle's concept of force seems wrong by Newtonian standards, but is actually reasonable in the light of relativity, as I previously argued.

But Einstein did not believe that gravitational acceleration was caused by the geometry of spacetime, as most physicists do today.

Einstein is also making a false distinction between gravity and electromagnetism. The preferred relativistic view of electromagnetism, as developed by Poincare, Minkowski, and maybe Weyl, is that the field is a curvature tensor and the force is a geometrical artifact. In this view, electromagnetism and gravity are formally quite similar.

In his paper on General Relativity from 1916 he writes: “the law of causality makes a sensible statement on the empirical world only when cause and effect are observable.” Since a gravitational “force” was not observable, Einstein had eliminated it from his theory of gravitation and replaced it by the curvature of spacetime. ...

There are people who deduce that the law of causality is invalid because the theory of relativity made it obsolete. Einstein himself seems to have gone back and forth on the issue.

The idea is mistaken, whether it is Einstein's fault or not. Causality is the essence of relativity.

In this connexion it is historically interesting that only ten years later Einstein converted from these ideas which had led him to his most fundamental contributions to physics. Werner Heisenberg who had used the same philosophy for the derivation of his uncertainty relation, recalls a conversation with Einstein in 1926; Einstein: “You don’t seriously believe that a physical theory can only contain observables.” Heisenberg: “I thought you were the one who made this idea the foundation of your theory of relativity?” Einstein: “May be I have used this kind of philosophy, nevertheless it is nonsense.”

Here is where Einstein rejects positivist philosophy for which he is widely credited.

The main reason Einstein is credited for special relativity over Lorentz is for the greater emphasis on observables. But as you can see, Einstein disavowed that view.

The role of causality in physics presents a problem. Although physics is widely understood to aim at describing the causes of observable phenomena and the interactions of systems in experimental set-ups, the picture of the world given by fundamental physical theories is largely acausal: e.g. complete data on timeslices of the universe related by temporally bidirectional dynamical laws. The idea that physics is acausal in nature, or worse, incompatible with the notion of causality, has attracted many adherents. Causal scepticism in physics is most associated with Russell’s (1913) arguments that a principle of causality is incompatible with actual physical theories. For causal sceptics, insofar as causal reasoning is used in physics, it is at best extraneous and at worst distorts the interpretation of a theory’s content.

No, causal reasoning is essential to physics. The arguments of Bertrand Russell and other philosophers rejecting causality are nonsense.

2 comments:

Ok, for the sake of playing the 'non causality' game I'll ask: If you don't believe in causality, how did you find out? Because if you didn't initially know it wasn't true, then something must have happened, You were told, learned, discovered, observed, or derived something that allowed you to say causality is false. Basically, something 'caused' you to change your mind...about there being no causes. Hmm....how exactly would that be possible if there was no causality? Divine inspiration? Spontaneous knowledge? Cosmic awareness? Extra-dimensional causes?